linux-kernel - Re: [PATCH 06/21] powerpc: Avoid comparison of unsigned long >= 0 in __access

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Message-ID: <CA+7wUsx047shRJ92PJ+cvg24HNhMnYzM0t5zUdEOaSE00sXkSg@mail.gmail.com>
Date:   Mon, 26 Feb 2018 18:50:11 +0100
From:   Mathieu Malaterre <malat@...ian.org>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        LKML <linux-kernel@...r.kernel.org>,
        Paul Mackerras <paulus@...ba.org>,
        Jiri Slaby <jslaby@...e.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 06/21] powerpc: Avoid comparison of unsigned long >= 0 in __access_ok

On Mon, Feb 26, 2018 at 8:44 AM, Mathieu Malaterre <malat@...ian.org> wrote:
> On Mon, Feb 26, 2018 at 7:50 AM, Christophe LEROY
> <christophe.leroy@....fr> wrote:
>>
>>
>> Le 26/02/2018 à 07:34, Christophe LEROY a écrit :
>>>
>>>
>>>
>>> Le 25/02/2018 à 18:22, Mathieu Malaterre a écrit :
>>>>
>>>> Rewrite check size - 1 <= Y as size < Y since `size` is unsigned value.
>>>> Fix warning (treated as error in W=1):
>>>>
>>>>    CC      arch/powerpc/kernel/signal_32.o
>>>> In file included from ./include/linux/uaccess.h:14:0,
>>>>                   from ./include/asm-generic/termios-base.h:8,
>>>>                   from ./arch/powerpc/include/asm/termios.h:20,
>>>>                   from ./include/uapi/linux/termios.h:6,
>>>>                   from ./include/linux/tty.h:7,
>>>>                   from arch/powerpc/kernel/signal_32.c:36:
>>>> ./include/asm-generic/termios-base.h: In function
>>>> ‘user_termio_to_kernel_termios’:
>>>> ./arch/powerpc/include/asm/uaccess.h:52:35: error: comparison of unsigned
>>>> expression >= 0 is always true [-Werror=type-limits]
>>>>     (((size) == 0) || (((size) - 1) <= ((segment).seg - (addr)))))
>>>>                                     ^
>>>> ./arch/powerpc/include/asm/uaccess.h:58:3: note: in expansion of macro
>>>> ‘__access_ok’
>>>>     __access_ok((__force unsigned long)(addr), (size), get_fs()))
>>>>     ^~~~~~~~~~~
>>>> ./arch/powerpc/include/asm/uaccess.h:262:6: note: in expansion of macro
>>>> ‘access_ok’
>>>>    if (access_ok(VERIFY_READ, __gu_addr, (size)))   \
>>>>        ^~~~~~~~~
>>>> ./arch/powerpc/include/asm/uaccess.h:80:2: note: in expansion of macro
>>>> ‘__get_user_check’
>>>>    __get_user_check((x), (ptr), sizeof(*(ptr)))
>>>>    ^~~~~~~~~~~~~~~~
>>>> ./include/asm-generic/termios-base.h:36:6: note: in expansion of macro
>>>> ‘get_user’
>>>>    if (get_user(termios->c_line, &termio->c_line) < 0)
>>>>        ^~~~~~~~
>>>> [...]
>>>> cc1: all warnings being treated as errors
>>>>
>>>> Signed-off-by: Mathieu Malaterre <malat@...ian.org>
>>>> ---
>>>>   arch/powerpc/include/asm/uaccess.h | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/uaccess.h
>>>> b/arch/powerpc/include/asm/uaccess.h
>>>> index 51bfeb8777f0..fadc406bd39d 100644
>>>> --- a/arch/powerpc/include/asm/uaccess.h
>>>> +++ b/arch/powerpc/include/asm/uaccess.h
>>>> @@ -49,7 +49,7 @@
>>>>   #define __access_ok(addr, size, segment)    \
>>>>       (((addr) <= (segment).seg) &&        \
>>>> -     (((size) == 0) || (((size) - 1) <= ((segment).seg - (addr)))))
>>>> +     (((size) == 0) || ((size) < ((segment).seg - (addr)))))
>>>
>>>
>>> IIUC, ((2 - 1) <= 1) is the same as (2 < 1) ?????
>>
>
> The whole series was pretty mediocre, but this one was actually pretty
> destructive. Thanks for catching this.
>
>>
>> Note that I already try to submit a fix for this warning 3 years ago
>> (https://patchwork.ozlabs.org/patch/418075/) and it was rejected with the
>> following comment:

Tested again today with gcc 6.3.0 and gcc is still producing the
original warning (treated as error).

>> Again, I don't think Linux enables this warning.  What did you do to
>> produce this?  In any case, it's a bad warning that doesn't take macros
>> into account, and the answer is not to make the code less clear by hiding
>> the fact that zero is a special case.
>
> Right. I'll try to see how to make W=1 run without error with an
> alternate solution.

So the other alternative is to update a bunch of ppc32 defconfig(s)
with: CONFIG_PPC_DISABLE_WERROR=y.

Would that be preferable ?

>> Christophe
>>
>>
>>>
>>> Christophe
>>>
>>>>   #endif
>>>>
>>